Molecular "light switches" can reveal exactly which neurons are involved in creating a memory, allowing scientists to trigger that memory using only light. The finding, presented at the Society for Neuroscience conference in Chicago this week, is just one example of how a novel technology called optogenetics is allowing scientists to tackle major unanswered questions about the brain, including the role of specific brain regions in the formation of memory, the process of addiction, and the transition from sleep to wakefulness.

The technology, developed just four years ago by Karl Deisseroth, a physician and bioengineer at Stanford, and Ed Boyden, now a bioengineer at MIT, is already being used by hundreds of labs across the globe. Thanks to molecular tinkering and new fiber-optic devices that deliver light deep into the brain via an implant, researchers can use optogenetics to study the effect of neural stimulation on different behaviors in live animals.

To make neurons sensitive to light, scientists genetically engineer them to carry a protein adapted from green algae. When the modified neuron is exposed to light, via the fiber-optic implant, the protein triggers electrical activity within the cell that spreads to the next neuron in the circuit. The technology allows scientists to control neural activity much more precisely than previous methods, which generally involved delivering electrical current through an electrode.

Michael Hausser's team at University College London is using optogenetics to probe how memories are stored in the brains of mice. According to the basic model of memory formation, learning a new association--such as that a particular sound precedes an electric shock--activates a subset of neurons in part of the brain called the hippocampus. "It's thought that recall of the memory can be triggered by activating only a subset of the cells in that network," says Hausser. "But there's no clear, direct experimental evidence for any of the steps in the process."

Hausser and his collaborators genetically engineered the light-sensitive protein so that it would only be expressed in neurons in the hippocampus that were activated during the formation of a memory. Next, they taught the mice to fear a particular sound by pairing it with an electric shock. Hearing the sound then made the animals freeze in fear--and triggered production of the protein in activated brain cells.

The next day, researchers shone blue light on the animals' hippocampi. That triggered activity in only the subset of cells that fired during memory formation the day before, causing the animal to freeze in fear in response to light, rather than to the sound. The researchers also labeled these cells with a fluorescent marker, allowing them to count the number of cells involved in the creation of the memory. "A remarkably small number of neurons in these animals [are] sufficient to drive recall, on the order of 100 to 200 cells," says Hausser.

In addition to illuminating the most basic aspects of the brain, researchers are using the technology to better understand specific maladies such as depression, Parkinson's, and addiction, in hopes of improving treatments. Parkinson's disease, for example, can be treated using deep brain stimulation, in which a surgically implanted electrode delivers pulses to a specific structure deep in the brain. But the procedure is invasive and carries risk of side effects such as depression and cognitive dysfunction. Earlier this year, Deisseroth's team published details of research using the light switches to study the brain circuits involved in Parkinson's disease. They found that they could alleviate the motor deficits in animals with Parkinson's-like symptoms by activating neural targets much closer to the surface of the brain.